Cellulose is the most abundant natural polymer on Earth and it is composed of linear chains of 1,4-D-glucopyranose units. It exists in nature in various forms, but primarily as the only component of cellulose nanofibers (ca. several micrometers long and 5-20 nanometers wide structures) which in turn are the major component of cellulose fibers (ca.1-4 milimeters long and 20-50 micrometers wide), the fibrillar component of higher plants.
Modern Materials Science has developed many examples of low density materials able to provide specific features. A number of products exist today based on different types of low-density cellulosic materials (also known as cellulosic foams or aerogels) bearing different functionalities. Foam compositions cover a wide range of products such as sponges, thermal and acoustic insulation materials, packing materials, personal care or medical products to cite only a few. In addition to the processing costs, the key parameters to make it useful for either application are the physical properties (density, structural consistency, absorbency, surface energy . . . ) and the functionalities that it bears (fire performance, biocompatibility, bioactivity, selective solvent affinity, etc.).
One well-documented strategy for obtaining low density cellulosic materials is adding porogens (insoluble particles like trisodium phosphate, sodium sulfate or polyethylene glycol, that are later leached out to produce pores) to a cellulose solution; U.S. Pat. No. 3,261,704 describes the basis of this process, while U.S. Pat. No. 3,261,704 provides examples of using porogens. Another cellulose foam creating strategy, changes the porogens for blowing agents as the way to create pores:
WO2014011112A1 provides examples of this. Nearly all these products using either porogens or blowing agents are made from a viscose starting material (cellulose, originally fibrous, that has been chemically converted into a thermoplastic polymer). Disadvantages of using viscose include the long preparation process, the high purity cellulose required and the severe environmental discharges produced by the process.
There are also a number of processes developed to produce low density cellulosic materials out of cellulose fibers. Some approaches are based on the use of organic solvents which are finally removed through evaporation (e.g. CA2810627A1; WO0146297A2). Greener approaches use aqueous systems which are then removed either by freeze-drying (e.g. US2014134088A1, WO2012032514; Jin et al. Colloid Surface A 240, 63, 2004) or drying under supercritical conditions (US2014079931A1; Gavillon and Budtova, Biomacromolecules 9, p. 269, 2008). Both strategies look to avoid the presence of liquid water between the highly hydrophilic cellulose fibers responsible of promoting strong capillary attraction between them (upon drying) what in turn densifies the final product. Both techniques require pressurized reactors with excellent temperature control, and thus are expensive and very difficult to industrialize.
Moreover, both freeze-drying and drying under supercritical conditions give rise to products with low resistance to water which easily lose their 3D structure (since the fibers are held together by highly water sensitive hydrogen bonding) and moderate mechanical performance. Since these characteristics limit their potential applicability in some cases, sometimes these products are later improved with extra treatments, such as chemically crosslinking cellulose fibers with specific reactants (Chinga-Carrasco and Syverud, J Biomater Appl 0, p. 1, 2014) which, in addition to raising the cost, affect negatively the recyclability of cellulose.
Cellulose- and especially cellulose fiber-based materials typically require strong functionalization and/or additivation in order to overpass cellulose inherent limitations in terms of water resistance, fire performance, biological attacks, compatibility with polymeric matrices, biocompatibility, etc. “Functionalization” implies that a substance interacts with the cellulose fibers creating stable bonds, such as covalent, ionic, hydrogen bonding, van der Waals, etc. On the other hand, “additivation” may refer to substances that are just retained or entrapped by the cellulosic structure. However, due to the bad resistance to water and instability of the 3D structure mentioned before, existing cellulose products (especially foams) are very limited to post-treatments in general and more particularly to water-based post-treatments. Accordingly, some very interesting water-based modification techniques, such as the layer-by-layer technique which is typically driven by electrostatic interactions, cannot be applied.
In view of the above exposed there is the need in the state of the art of providing new cellulosic products with improved properties, such as the water stability, so that they can be easily chemically modified by post-treatment, especially in water-based media, to various features and different grades, in order to reach as many potential applications as possible.